[TowerTalk] ZL-Special

[Rod Brink]

To Brian Harris who asked about my ZL-Special:

Unfortunately I've recently moved and a lot of my stuff is in storage,
including my notes on that antenna.  However, I'll tell you what I remember.
The two radiating elements were parallel, half-wave folded dipoles, each
about 25' 8" long, constructed of 300 ohm TV twinlead.  The phasing line
that interconnected them at their midpoints was 6' 8" long, also twinlead,
and was given a half-twist.  This put the two dipoles about 125 deg out of
phase from each other and gave the antenna its directionality and
front-to-back (which was about 18-20db.  My modeling software says that by
playing around a little with the length of the phasing line, its possibly to
achieve theoretically infinite front-to-back).
The feedpoint impedance came out to be 35 +j30, as I remember.
I wound a toroidal 75-to-50 ohm transformer and connected it in parallel
with a 300 pf mica cap at the feedpoint to transform the 35 ohms to 50 ohms,
and to cancel out the inductive reactance.

Physically, I suspended the antenna between aluminum conduit pipes 7' long
at the ends in order to maintain the spacing.  Matching network was built
into small plastic boxes I got at Radio Shack, and partly filled with epoxy
for water-proofing.  I hung it horizontally, like a Yagi, at about 50'.  I
used RG-58 to hang down to the ground to minimize sag, then switched to
RG-8.

Although  it wasn't the classiest looking thing in the world, it was a
KILLER antenna.

73, Rod




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